Reply to the discussion by Grimstad et al. on “Viscous behaviour of clays in one-dimensional compression”

Abstract

We, the authors of this paper, wish to express our appreciation to Grimstad, Degago, and Boumezerane for their time and effort in studying our paper and preparing the Discussion. We view their Discussion to be equally important as our paper. Their comments address (i) the uniqueness of the void ratio at end of primary consolidation, eEOP, (ii) the creep deformation of individual particles, and (iii) the concept of isotache. Themain focus of the paper is to identify different deformation modes involved in one-dimensional (1D) creep compression. One of the main findings of our study is that creep compression deformation at the contacts causes compression of the large interaggregate pores, which is a continuation of primary consolidation. During primary consolidation, most of the excess pore pressure generated by the total stress change has dissipated. The eEOP is proposed and used as a reference state to illustrate the continuation of the collapse of the large interaggregate pores. Our experimental data illustrate that the normal consolidation line (NCL) is unique, implying the uniqueness of eEOP based on the assumption that the time required for consolidation is small, i.e., the time-dependent deformation during consolidation and the excess pore pressure induced by the time-dependent deformation are insignificant. We concur that time-dependent deformation occurs in the entire compression process involving primary and secondary compression. One can argue the uniqueness of the coefficient of consolidation and the NCL if the time-dependent deformation dominates in the primary consolidation. The major challenge is that the 1D consolidation test is not an element test, and the response of the test sample is dependent on the initial and imposed boundary conditions. The intrinsic properties are very difficult to determine from the test results without any assumptions. In soft geomaterials such as soft clay and peat, it is challenging to separate or decouple the creep deformation process from the consolidation. In such cases, we propose the use of nondestructive methods such as microcomputer tomography scanning and nuclearmagnetic resonance imaging tomonitor the evolution of the soil structure at a microlevel during 1D compression. We also agree that the creep deformation of individual particles is very small in normally consolidated clay at low stress levels. However, this mode of deformation becomes dominant in heavily overconsolidated clay (clay shale) and sand at high stress levels. The concept of isotache is one of empirical correlations used in modeling the time-dependent behaviour of soils. However, it has some limitations in explaining the behaviour observed in relaxation tests in which the strain rate in one direction is zero.